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 Post subject: Selenium Rectifier Testing
PostPosted: Jan Sun 04, 2009 1:54 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
I have removed the selenium rect from my radio and replaced it with the 1N4007 diode. I have yet to power up the radio. I decided to check the old selenium rect, and found that it conducts in both directions with my meter set to ohms. It tested 1.7mohms in one direction and 3.7mohms with leads reversed. Is the sel/rect bad and would this cause abnormal (high)voltage to the filament string. I am trying to get up to speed so could someone explain. I know that the rect. should be a half wave rect I guess? Maybe?


MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 2:44 pm 
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Joined: Feb Mon 04, 2008 3:20 am
Posts: 562
Location: Grand Rapids MI
Hi Mike!!

You don't say what meter your using to check the rectifier, and that does make a difference because they don't all use the same voltage on higher scales. What you're seeing is normal. Check a diode and you'll see the same thing. When selenium rectifiers fail the voltage drop through them goes way high resulting in lower voltage to the set. Yes the rectifier is half wave. You don't say what set your working on, but it is possible that after the new diode is installed that the filament voltage may be (almost always is) higher than it should be so I would check that with the tubes out of the set. You can do some calculations (add the tube voltages together) to determine what the filament voltage should be at the first tube and go from there. If its a series string set, then it has a dropping resistor to get to the proper value of voltage and all the tubes should draw the same current. You can use the voltage drop of the tubes, add them together to determine total drop and then, using an older meter that loads the circuit, get a ballpart figure of voltage a t the first tube in the string. It should probably measure 15 to 20 percent high because of no load on the circuit. A better way is to look at the tube manual for current load of on of the tube filaments, figure the voltage drop of the entire tube string, and build a resistor that approximates the load and tack it into the circuit, fire up the set and measure the voltage present at the resistor. It should show the voltage required for the string of tubes. If its high, then adjust your dropping resistor value so that it's correct. Does this help??

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W8ZV
kim.herron@sbcglobal.net
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 Post subject:
PostPosted: Jan Sun 04, 2009 3:23 pm 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 5426
Location: Leo, IN or Zellwood, FL
What he said sorta. Best Mike to come back and tell us what radio your working on. If it is a Zenith TO with the filament voltages developed from the cathode curcuits of the tubes. Then do NOT
try to power it up without tubes.
The Diode 1N4007 will give you a bit higher B+ voltage than a brand new Selenium Rectifier. In some radios you will want to add a series resistor to the diode to get this voltage back to correct number.

John k9uwa/w4 snowbird


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 Post subject: Selenium Rect. Testing
PostPosted: Jan Sun 04, 2009 4:35 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
The radio I am working on is a Zenith T/o Y600L. I was getting 38vdc at the first tube in the string, book states voltage should be between 8-9vdc. The meter I am using is a Fluke 73 series2 multimeter. I tested the diode and it tested the way I would think it should test. I took the first tube in the string out, powered up the radio to get a voltage reading. Based on what some members are saying I could have a problem when I reinstall the tube. Is this correct? Should I try to discharge the filter caps?

MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 5:09 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Yes, by all means discharge them before plugging the tubes back in! If you are only getting 38 volts with no load, then I would have to agree that the rectifier was bad.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject: Re: Selenium Rect. Testing
PostPosted: Jan Sun 04, 2009 5:20 pm 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 5426
Location: Leo, IN or Zellwood, FL
Don't RUN A ZENITH TO WITHOUT ALL THE TUBES IN IT.

Best also if you have one or get one... a 1R5 tube with a pin cut
off and put it in place of the 1L6 tube for all your testing.

See pricing of 1L6 tubes and you will understand why.

Voltages will NOT read correctly without all the stuff in place in this radio.

John k9uwa /w4


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 Post subject:
PostPosted: Jan Sun 04, 2009 6:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 31490
Location: Livermore, CA
MikeO

Do not operate your radio with a tube missing. Never pull or replace a tube in this radio with power on. You will blow at least one tube filament. Done it many times...

With tubes in the radio there is a load on the filament filter cap. This keeps voltage around 9. With a tube removed voltage will go way up across this cap. If you were to replace a tube with the cap charged one or more tube filaments will burn out.

Reading 38 volts shows a tube filament may be blown? You can check continuity of each tube filament by measuring resistance between pin 1 and 7. These are pins each side of the gap. Do this with tubes removed from the radio. Be sure the radio is unplugged when removing tubes.

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Norm


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:15 pm 
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Joined: Dec Tue 16, 2008 4:50 pm
Posts: 33
I checked all tubes pins 1 and 7 and they all test good. I was told that the first number in the tube designation indicates the operating voltage for that tube. Is that correct? If that is true, I have 5 tubes in my filament string, 3V4,1U4,1L6,1U4,1U5. Doing the math I get 7vdc. The book calls for 8-9vdc at pin 7 of the first tube. I guess what you are saying is without a load this voltage would be much higher. Also, in testing voltages along the string, there should be a voltage drop across each tube plus any dropping resistors such that at the last tube the voltage should be 0. That seems to be what I am getting. Please forgive my stupidity, I'm not that bright but I work hard. Another thing, I am in the process of recapping this radio(Zenith T/O Y600L) and I have found all of the leads that are indicated by symbols but cannot find the leads to the 200uf cap which has no symbol. the filter can seems to only have 4 leads coming out. a 60uf,20uf,40uf and one unmarked lead which is brown in color which I took for the common ground for the caps or B-




MikeO


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:18 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
The filament voltages designated by the first digit of the tube number are rounded off values. For the tubes starting with 1, the actual filament voltage is 1.4 volts, and for the tube with the 3 prefix, the actual voltage is 2.8 volts.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:24 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 31490
Location: Livermore, CA
MikeO

You are correct. The first number indicates filament voltage but instead of 1.5U5 they used 1U5. It's actually a 1.5 volt tube, like others starting with 1.

Each tube should have 1.4 - 1.5 volts across filament pins when operating in your radio. If it is mush less the oscillator (1L6) won't operate. If one tube reads real high. It's either bad or has poor socket contact.

Your filter capacitors are in a metal can. The metal can is common negative for all 4 sections. Symbols are shown on the schematic. The 200mf section isn't marked.

http://www.transoceanic.nostalgiaair.org/600/y600.pdf

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Norm


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 Post subject:
PostPosted: Jan Sun 04, 2009 7:38 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 6259
Location: Santa Rosa, CA
Norm: good advice.

MikeO, you might want to "google" for Transoceanic restoration tips. There are loads of useful pages. Here is one:

http://oak.cats.ohiou.edu/~postr/bapix/H500_2.htm

You can kill your tubes easily by high filament voltage or by changing them while the circuit is energized or the capacitors are charged.

Also, it's not clear that a VOM or DMM reading will tell you much about a selenium rectifier's condition. Unless the measurement circuit in the meter uses sufficient voltage to overcome the inherently high forward voltage drop of the selenium unit, it won't give a meaningful reading. The best way would be to measure the DC voltage across the selenium rectifier terminals with the rectifier out of the circuit and powered up by a small external power supply (maybe about 20 volts). A good selenium might be between 5 and 10 volts.

Changing to a silicon diode WILL increase the voltage on the capacitors and the tube filaments, unless you add a series resistor to compensate.

Rich


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 Post subject:
PostPosted: Jan Sun 04, 2009 8:21 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7804
Location: Minnesota
First of all you cannot test a selenium with a ohmeter, they must be tested under a specified load for voltage drop, the junction in a selenium does not form until current is applied. They are nothing like a silicon diode and cannot be tested the same way. Very seldom will you ever find a bad selenium in a TO.

Second, read everything you can on the restoration of TO's, you will save yourself a lot of trouble and blown tube filaments. The very first step is to replace the electrolytics and capacitors.

Dave


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 Post subject:
PostPosted: Jan Tue 06, 2009 9:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4271
Location: Woodinville, WA USA
I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can :)

Many people who restore TransOceanics add a power resistor (wirewound, etc.) in series with the new diode. Something rated for 2 or more watts, in the range of 47 to 180 ohms. If you are fussy, you can experiment with different values until your voltages look just right.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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 Post subject:
PostPosted: Jan Wed 07, 2009 12:31 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 31490
Location: Livermore, CA
Hi

A selenium rectifier is made up of several diodes. Each plate is one diode which are in series.

A diode has a specific voltage drop. All, usually 6 in series, add up to 7 volt drop on a good selenium rectifier. Meters use a 1 1/2 volt battery on ohm scale. This isn't enough to overcome the selenium rectifier drop. Some meters have a higher voltage battery on the meg ohm scale. Problem with this reverse direction of a selenium rectifier has enough leakage so there is a reading in either direction.

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 Post subject:
PostPosted: Jan Wed 07, 2009 2:52 am 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 5426
Location: Leo, IN or Zellwood, FL
[quote="philsoldradios"]I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can :)

Phil Nelson

DITTO There is no such thing as a "good" Selenium Rectifier. Even a NOS one should be as Phil says.. Clanged into the Round File.

Besides they stink like crazy when croak.. plus the gas given off is claimed to... cause cancer.

John k9uwa /w4


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 Post subject:
PostPosted: Jan Wed 07, 2009 4:40 am 
Member

Joined: Oct Sun 07, 2007 5:22 am
Posts: 947
Location: Cascadia
Phil wrote
Quote:
I test selenium rectifiers by listening for the satisfying clang when I pitch them into the garbage can Smile


Yes, but they are part of the original radio's heritage. I never remove them if I can avoid it. On a radio or TV I plan to use a lot I might bypass the selenium rectifier and hide a modern diode under the chassis or elsewhere. But throw them away? Never.


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